A primary objective in this research is to broaden our understanding, from both basic science and clinical perspectives, of the role of the muscles of mastication in influencing the stability of surgical results in both growing and nongrowing individuals. More specifically, this study will emphasize the effect of maxillary osteotomy and increase in vertical dimension of the midface on the growth and remodeling of the maxillomandibular complex using both juvenile and adult Macaca mulatta as experimental models. Two specific experimental studies - one in adults and one in juvenile - are proposed to evaluate the effect of lengthening of the mandibular elevators on the short-term and long-term stability of the maxilla following total maxillary osteotomy and interpositional graft to increase midfacial height. Based upon previous studies in our laboratory, the following specific hypotheses, or predictions, are put forward. a. Surgical detachment and reattachment of the masseter muscle at the time of maxillary osteotomy and vertical augmentation reduces relapse through the use of a bite-splint preoperatively reduces relapse tendencies. c. Vertical relapse in adults who are not preadapted to a lengthened musculature continues over a year-long period through a combination of resorption of the osseous graft, initially, and distraction of the maxilla superiorly and anteriorly and intrusion of the teeth after bone healing is complete. d. Vertical relapse in juveniles occurs only during the first 12 weeks after surgery, following which time the maxillomandibular complex resumes its normal downward and forward displacement during growth. Cephalometrics with the aid of radiopaque bone and muscle markers, bite force analysis, and EMG will be analyzed in a longitudinal basis to evaluate the hypotheses. Histochemistry and histology of muscle fibers, the TMJ, and the graft sites will also be performed.